Due to the increase in length and flexibility of rotor blades in wind turbines, the controllers of these are having to work increasingly hard to measure the aerodynamic loads of one or a plurality of rotor blades occurring during operation. Loads or deformations of the rotor blades are mainly caused by aerodynamic forces. Direct measurement of the aerodynamic loads is not possible, however. Since the bending of the blade and the twisting of this and the blade load are closely related to one another, information on these undesired deformations forms a useful starting point to determine whether these deformations are for example due to a misalignment of the rotor axis in relation to the wind direction or to vertical or horizontal wind shears.
The measurement of the loads of a rotor blade during operation usually takes place using sensor devices, which are arranged in a cavity of the rotor, or of the rotor blade or also in a fixed machine housing (gondola) of the wind turbine. From a signal source signals are sent to a reflector arranged at a distance from this, which are then reflected by this and sent to a receiver similarly arranged at a distance from the reflector. In the receiver, the deflected signals are evaluated and if necessary converted by a processing unit into measured values. The receiver and processing unit are referred to in the following as the signal evaluation device.
The spatial displacements of markings on the reflector surface caused by the aerodynamic loads can thus be detected and quantified by the signal evaluation unit. The signal evaluation unit can, for example, be a so-called non-contact distance measuring device in the form of a laser, a camera or similar arrangements. The evaluation device can be arranged either in the cavity of the rotor, or of the blade or even by use of suitable transmission means in the fixed machine housing (gondola) of the installation. The signals can be optical, sonar, laser or electromagnetic signals. The reflector is usually a so-called retroreflector, which is arranged symmetrically about the blade axis and secured to the blade.
Examples of such generic sensor devices with a reflector and a signal evaluation device in the cavity of a rotor blade are described in EP 2 239 462 A2 DE (non-contact distance measuring device), DE 10 2009 007 938 A1 (optical sensor) or WO 2010054661 A2 (signal evaluation device in the gondola).
There are two influencing variables that have a substantial effect on the accuracy of the measurements of the sensor devices described above in the cavity of the rotor area of the wind turbine: firstly, the movement of the signal evaluation devices caused by the aerodynamic loads, e.g. with an optical system, if the camera itself moves due to the load, and secondly due to soiling of or damage to the reflector during operation.
Any movement of the signal evaluation device can be prevented by fixed positioning of the device or compensated by a reference point measurement. With reference point measurement, by means of an additional measurement of a distance to a fixed point (for example the distance to the blade root) the movement of the signal evaluation device is calculated. In this regard see the as yet unpublished German patent applications DE 10 2010 017 749.0 and DE 10 2011 011 392.4 of the applicant.
The second influencing variable concerns possible soiling of and damage to the reflector in the rotor area of the wind turbine. This area is relatively inaccessible. This is in particular true of the cavity of the rotor blade, which narrows towards the blade tip. The reflector of the sensor device is arranged at a distance from the blade root in the direction of the blade tip in this inaccessible cavity of the blade.
Soiling of or damage to the reflector are caused by dust, deposits of dirt or detached parts flying around in the rotor area, which can have various causes. These may be residues from the blade manufacture, machine parts coming loose or even tools. On the reflector surface over time dirt deposits accumulate from machine oil or other dirt particles. These deposits cause a deviation in the incoming and reflected signals, resulting in the signals detected being wrong and incorrect measurements being recorded and quantified.